In general, a clathrate hydrate means crystalline compounds that guest molecules are physically trapped inside a three dimensional lattice structure formed by a hydrogen bond of host molecules, without chemical bonds. Also, a gas hydrate means that host molecules are water molecules and guest molecules are low molecular gas molecules such as methane, ethane, propane, or carbon dioxide.
The clathrate hydrates were first discovered in 1810 by Sir Humphry Davy of England. In a Baker Lecture targeting the Royal Society, he announced that when chlorine is reacted with water, a compound similar to ice is formed, but its temperature is higher than 0° C. Also, the fact that the gas hydrate is produced when one chlorine molecule is reacted with ten water molecules, was first discovered in 1823 by Michael Faraday.
Thereafter, until now, researchers continue to study this gas hydrate as one of phase change materials, in which their principal research fields include a phase equilibrium and generation/dissociation condition, crystal structure, coexistence phenomena of polycrystal, competitive change of composition in a cavity, etc. In addition, detailed researches are still progressing at various macroscopic and microscopic views.
Of the guest molecules which are able to be trapped in the gas hydrate, 130 kinds of guest molecules or more are known so far, for example, CH4, C2H6, C3H8, CO2, H2, SF6, etc. Also, gas hydrate crystal structures, which are constructed by a polyhedron-shaped cavity formed by water molecules having a hydrogen bond and based upon a formation condition and a kind of gas molecule, have the following crystal structures: a body-centered cubic structure I, sI; a diamond cubic structure II, sII; and a hexagonal structure H, sH. The sI and sII are determined by the size of the guest molecule, and in the sH, the size and the shape of the guest molecule function are important factors.
The guest molecule of gas hydrate is trapped in deposits of methane on the deep ocean floor or frozen soil, and such methane is in the spotlight as a clean energy source because a small quantity of CO2 is created during its combustion. More specifically, the gas hydrate can be used as an energy source possible to substitute the existing fossil fuels, and may be used to facilitate the solid storage and transportation of natural gas by utilizing the hydrate structure, or for isolation/storage of CO2 to combat global warming. Also, it may be used for an apparatus for separating gas or aqueous solution from it, such as, in particular, a seawater desalination apparatus.
The gas hydrates are mainly found in petroleum or natural gas reservoirs, an area adjacent to a coal bed, or a sedimentary layer in deep-ocean having a condition of a low temperature and a high atmospheric pressure such as, in particular, a continental slope.
Also, the gas hydrates may be artificially produced and generally have a shape as shown in FIG. 1.
FIG. 1 illustrates a typical gas hydrate producing apparatus 10 as described in a prior art.
The typical gas hydrate producing apparatus 10 described in the prior art includes a water supplying portion 1, a gas supplying portion 2, a reactor 3 in which water from the water supplying portion 1 is reacted with the gas supplying portion 2, an ejector 4 for ejecting to the outside of the gas hydrate generated by the reactor, and a stirring apparatus 5 to increase the reaction rate between water and gas. To establish an environment within the reactor 3 to have an appropriate temperature condition while producing the gas hydrate, a separate cooling jacket 6 may be provided to surround the outside of the reactor 3. The cooling jacket 6 is connected to a refrigerant supplying portion 7 so that a refrigerant can be continuously supplied.
The reactor 3 for forming the gas hydrate has a cylindrical shape and is equipped with the stirring apparatus 5 having a stirrer or screw type for stirring altogether the water and the gas supplied into the inside of an apparatus for enhancing a reaction efficiency and an efficiency for elimination of heat of reaction. In the stirring apparatus 5 having such a cylindrical shape, the gas hydrate is essentially created, regardless of phases of supplied water and gas or a supplying method and type, and the generated gas hydrate is easily attached on the wall surface of the inside of the reactor 3 having a cylindrical shape. Once the gas hydrate is attached, it has a characteristic that it continues to grow by performing a role of nuclear seed by itself.
Due to such characteristics, since a kind of film or wall is formed on the wall surface of the inside of the reactor 3 after the lapse of time, it hinders a flow of fluid inside the reactor 3 and functions as a heat transfer wall. In particular, since a thermal conductivity of the gas hydrate is lower than that of ice, it leads to a strong insulation effect by hindering a thermal flow through the wall surface of the reactor 3. As a result, the reaction efficiency to the outside of the reactor 3 is remarkably reduced, which negatively affects the cost efficiency of a heat exchange. Preferably, in the reactor, it is necessary to quickly remove the heat of reaction outwardly.
Also, attempts have been made throughout the years to prevent forming of the gas hydrate on the wall surface of the inside of the reactor 3, for example, by utilizing a process of generating a strong disturbance inside the reactor 3 or a process of coating a thin hydrophobic film on the inner wall surface of the reactor, etc.
However, conventional solutions have not been able to effectively control an attachment and a growth of the gas hydrate on the wall surface of the inside of the reactor.